In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, use of a new frequency band or improvement in frequency utilization efficiency in LTE (Long Term Evolution) has been investigated in order to satisfy increasing traffic demand (for example, see Non-Patent Literature 1).
Specifically, in 3GPP, technology to enable a frequency band (unlicensed band) not requiring a license, which is used in the Wi-Fi (registered trademark, the same applies hereinafter), the Bluetooth (registered trademark, the same applies hereinafter) or the like, to be available in an LTE system (Licensed Assisted Access (LAA)) has been investigated. In LAA, carrier aggregation is applied to a licensed cell (Licensed LTE Cell (L-Cell)) using a licensed band requiring a license and an unlicensed cell (Unlicensed LTE Cell (U-Cell)) using an unlicensed band.
When a signal is transmitted by using an unlicensed band, by law (for example, the Radio Law in Japan), it is defined that a Listen before Talk (LBT) scheme using carrier sensing is based before transmission of the signal, in consideration of effect on communication by another mobile communication provider, such as the Wi-Fi which has been already operated.
On the other hand, in a conventional LTE specification, a user terminal (UE) needs to acquire configuration information of a secondary cell (SCell) and synchronize with the SCell in order for carrier aggregation. In the conventional LTE specification, the configuration information of the SCell is notified to the UE by a control signal from a base station (eNB) through a primary cell (PCell). For synchronization of the SCell and the UE, a synchronization signal transmitted from the eNB through the Scell is used.